This invention relates to a spin size and thermosetting aid for pitch fibers.
In order to convert pitch fibers into carbon fibers it is necessary to first thermoset them before they can be carbonized to produce the desired final product. Generally, such fibers are spun and further processed into carbon in the form of multifilament yarn or tow. Because of the exothermic nature of pitch oxidation, however, hot spots often develop in the multifilament bundle during thermosetting which cause the fibers to melt or soften before they become infusibilized. As a result of this, deformation of the individual filaments occurs along with exudation of molten pitch through the filament surfaces which causes them to stick together at various points of contact along the length of the yarn or tow. This deformation and sticking of the fibers in turn causes the yarn or tow to become stiff and brittle and to suffer a loss of flexibility and tensile strength. As a result, such yarn or tow cannot be further processed without breaking a large number of filaments.
Spin sizes are conventionally applied to pitch fiber yarn or tow immediately following spinning in order to maintain the integrity of the yarn or tow, to provide lubricity at the filament-to-filament interfaces, and to impart abrasion resistance to the filament bundle. However, while such sizes improve the handleability of the yarn or tow prior to thermosetting, they often are of no value, or only of limited value, during thermosetting. Thus, for example, while mixtures of plain water and glycerol impart good handling properties to as-spun pitch fiber yarn or tow, such yarn or tow is still subject to the same disadvantages encountered during thermosetting of unsized yarn or tow, i.e., melting and sticking of the fibers often occurs which causes a reduction of the flexibility and tensile strength of the fiber bundle.
One attempt to overcome the sticking problem encountered during thermosetting is disclosed in U.S.S.R. Pat. No. 168,848. The approach to the problem suggested in that reference is to fan the filaments with coal dust prior to thermosetting. However, not only is this method dirty and inconvenient, but it is also very difficult to apply a uniform layer of particles to the filaments by this technique. Furthermore, because coal has a high inorganic impurity content, significant pitting of the fiber surfaces occurs during oxidation which is accompanied by a concomitant reduction in the strength of the fibers after carbonization.
A similar attempt to surmount the sticking problem and at the same time accelerate oxidation of pitch fibers is disclosed in U.S. Pat. No. 3,997,654 wherein it is suggested that the fibers be dusted with activated carbon which has been impregnated with an oxidizing agent. However, this procedure appears to suffer from the same disadvantages as the process of U.S.S.R. Pat. No. 168,848. Furthermore, because of the hardness and large size of the particles employed (60 microns), this procedure does not provide sufficient separation of the filament bundle to allow maximum contact of the oxidizing gas with the fiber surfaces or provide sufficient lubricity between the fibers to prevent physical damage to the fiber surfaces.